Field
The disclosure discussed herein relates to a glass substrate for a mask blank, a mask blank, a photomask, and a method for manufacturing a glass substrate for a mask blank.
Description of Related Art
A mask blank includes a glass substrate and a film formed on a main surface of the glass substrate. The number of the film may be one or more films. The film may be formed by overlapping multiple films. A photomask is obtained by forming a circuit pattern on at least one film. In a semiconductor exposing process, a circuit pattern of a photomask is reduced in size and transferred to a wafer. The main surface of the glass substrate is desired to have a high flatness, so that positional deviation of the circuit pattern (hereinafter also referred to as “pattern overlay accuracy”) is within a permissible range. A center area of the main surface of the glass substrate is particularly desired to have high flatness because a circuit pattern is formed in the center area of the main surface of the film on the glass substrate.
Processes such a polishing and local processing (see, for example, Patent Document 1) are performed for improving the flatness of the main surface of the glass substrate. In the first example of Patent Document 1, a hard polishing pad is used in a rough polishing step whereas a polishing pad having a porous resin layer (referred to as nap layer) is used in a fine polishing step. The 100% modulus of the resin of the porous resin layer is greater than or equal to 14.5 MPa.